ASASSN-18ey: The Rise of a New Black Hole X-Ray Binary
Kochanek, C. S.; Rest, A.; Stalder, B.; Smith, K. W.; Denneau, L.; Weiland, H.; Huber, M. E.; Flewelling, H.; Stanek, K. Z.; Dong, Subo; Thompson, Todd A.; Shappee, B. J.; Tucker, M. A.; Tonry, J. L.; Heinze, A. N.; Holoien, T. W. -S.; Prieto, J. L.; Beacom, John F.; Strader, J.; Chomiuk, L.; Auchettl, K.; Bahramian, A.; Rowan, D. M.; Shields, J.; ASAS-SN; ATLAS; Dage, K.
United States, Denmark, Australia, China, Chile, United Kingdom
Abstract
We present the discovery of ASASSN-18ey (MAXI J1820+070), a new black hole low-mass X-ray binary (LMXB) discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). A week after ASAS-SN discovered ASASSN-18ey as an optical transient, it was detected as an X-ray transient by MAXI/GCS. Here, we analyze ASAS-SN and Asteroid Terrestrial-impact Last Alert System pre-outburst optical light curves, finding evidence of intrinsic variability for several years prior to the outburst. While there was no long-term rise leading to the outburst, as has been seen in several other systems, the start of the outburst in the optical preceded that in the X-rays by 7.20 ± 0.97 days. We analyze the spectroscopic evolution of ASASSN-18ey from pre-maximum to >100 days post-maximum. The spectra of ASASSN-18ey exhibit broad, asymmetric, double-peaked Hα emission. The Bowen blend (λ ≈ 4650 Å) in the post-maximum spectra shows highly variable double-peaked profiles, likely arising from irradiation of the companion by the accretion disk, typical of low-mass X-ray binaries. The optical and X-ray luminosities of ASASSN-18ey are consistent with black hole low-mass X-ray binaries, both in outburst and quiescence.